A study is proposed to analyze the attention deficits of children aged 8 to 16 years carrying the dual diagnoses of Mental Retardation and Attention Deficit Hyperactivity Disorder. In this pilot study, MR children with ADHD will be compared to MR children without ADHD on laboratory measures of attention that have previously been used to assess attention in non-MR children of similar mental ages to the participants in this study. These laboratory measures will assess sustained attention, impulsivity, and selective attention. Results are expected to indicate the pattern of attention deficits associated with ADHD in mental retardation. Since it is hypothesized that mental retardation and ADHD are not orthogonal to each other, the pattern of attention deficits in MR ADHD subject relative to their non-ADHD MR peers is predicated to be different from the pattern of attentional deficits noted in ADHD children of normal intelligence relative to their hon-ADHD peers. One long-term objective of this proposed study is to provide measures that will be used in future research investigating the basis of the differences in cognitive processes between MR and non-MR children. A second long-term goal of this study will be to use these measures in future studies to assess the effectiveness of various dosages of medication upon both the cognitive and behavioral functioning of MR ADHD children.